Power control for hair iron having ceramic heaters

ABSTRACT

A hair iron includes two longitudinally extending arms each having a ceramic heater. Users place hair between the heaters during use. A controller coordinates heating conditions of the heaters. If a line voltage supplied to the hair iron is a high voltage, the controller is configured to alternate heating between the heaters whereby only heating of one heater occurs at one time. To reduce flicker and harmonics, the controller supplies heating current so that one heater remains off for consecutive half-cycles of alternating current while the other heater is powered on variously. The controller then switches the heating. The hair iron further includes a line voltage detector to detect whether a line voltage supplied through a cord to the hair iron is either low voltage or high voltage. During such time, the controller limits heating power to the two heaters.

BACKGROUND 1. Field of the Invention

The present disclosure relates to alternating current (AC) power control systems. More particularly it relates to power control methods and apparatus for controlling the AC power delivered to a hair iron having ceramic heaters.

2. Description of Related Art

Many conventional hair irons, such as flat irons, straightening irons, curling irons, crimping irons, etc., suffer from heat lag, which results in lengthy times to heat irons for use and cool them afterwards. During these times, users often set their irons on countertops or the like which creates a safety risk, especially to children, who may accidentally contact them while hot. Additional safety risks also arise when users accidentally leave irons powered on when not in use. The inventors recognize a need for hair irons having faster heating and cooling, which tends to improve safety.

Hair irons draw power from an electrical power grid to operate, i.e., alternating current (AC) line power. In various geographies, countries supply power at a relatively low voltage, e.g., 115 VAC, or high voltage, e.g., 230 VAC. Manufacturers design their irons to operate at one voltage or the other, but not both. When traveling between countries, users regularly carry with them voltage-conversion devices to appropriately change the voltage of the line power to match that of their hair iron. At either low or high voltage, however, irons can draw power in such a manner that they generate power harmonics and voltage flicker. In most locations, regulatory bodies set strict certification requirements that dictate (un)acceptable amounts of flicker, harmonics, current symmetry, radiation, conduction, and the like to reduce undesirable health effects on users and/or disruption to sensitive electronic/electrical equipment. As a result, manufacturers are continuingly challenged to meet requirements while not compromising temperature control performance and safety. The inventors further recognize a need for hair irons that users can conveniently use anywhere without requiring voltage converters, while also complying with flicker and harmonics requirements, such as those set by the International Electrotechnical Commission (IEC).

SUMMARY

The foregoing and other problems are solved by a hair iron having two longitudinally extending arms each with a ceramic heater. Users place hair between the heaters to heat the hair and style it. A controller coordinates heating of the heaters. If a line voltage supplied to the hair iron is a high voltage, the controller is configured to alternate heating between the heaters whereby only heating of one heater occurs at one time. To reduce flicker and harmonics, the controller supplies heating current so that one heater remains off for consecutive half-cycles of alternating current while the other heater is powered on variously. The controller then switches the heating to the other heater. The hair iron further includes a line voltage detector to detect whether a line voltage supplied through a cord to the hair iron is either low voltage or high voltage. During such time, the controller limits heating power to the two heaters. One or more thermistors provide temperature feedback to the controller.

In other embodiments, a hair iron includes a first arm and a second arm movable relative to each other between an open position and a closed position. A distal segment of the first arm is spaced from a distal segment of the second arm in the open position. The distal segment of the first arm is positioned in close proximity to the distal segment of the second arm in the closed position. A contact surface is positioned on an exterior, such as an exterior of the distal segment, of the first arm for contacting hair during use. The first arm includes a heater having a ceramic substrate and an electrically resistive trace on the ceramic substrate, e.g., on an exterior face of the ceramic substrate. The electrically resistive trace is composed of an electrical resistor material. In some embodiments, the electrically resistive trace includes the electrical resistor material thick film printed on the exterior face of the ceramic substrate after firing of the ceramic substrate. The heater is positioned to supply heat generated by applying an electric current to the electrically resistive trace to the contact surface. Embodiments further include those wherein the heater includes one or more glass layers on the exterior face of the ceramic substrate that cover the electrically resistive trace for electrically insulating the electrically resistive trace.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, is better understood when read in conjunction with the appended drawings. However, the invention is not limited to the specific methods and components disclosed herein. Like numerals represent like features in the drawings. In the views:

FIGS. 1A and 1B show a representative hair iron having power control for ceramic heaters;

FIGS. 2A and 2B are views similar to FIGS. 1A and 1B having covers removed to reveal electronic components;

FIG. 3 is a diagrammatic schematic view of a power control system of a hair iron;

FIGS. 4A and 4B are plan views of an inner face and an outer face, respectively, of a ceramic heater for use in a hair iron;

FIG. 5 is a cross-sectional view of the heater shown in FIGS. 4A and 4B taken along line 5-5 in FIG. 4A;

FIGS. 6A and 6B show methods of detecting AC line voltage and operating a hair iron therefrom;

FIGS. 7A and 7B show example waveforms illustrating changes in the half-cycles of AC line voltage and power control therefor;

FIG. 8 shows a Table for applying AC half-cycles of power to heaters of a hair iron to achieve desired temperature responses; and

FIGS. 9A and 9B show further AC waveforms illustrating changes in the half-cycles of AC line voltage and power control therefor to have no DC offset applied to the heaters.

DETAILED DESCRIPTION

It is to be understood that while the preferred embodiments herein incorporate AC power delivery for a hair iron, the principles and concepts can be utilized in many other applications. Applications that are especially well adapted for using the features of the invention include other small appliances where AC power is to be delivered to the appliance at different AC voltages and likely to cause flicker and the generation of harmonic energy. The features of the invention can be utilized with AC power systems having frequencies and voltages different from that used in the United States.

Referring now to the drawings and particularly to FIGS. 1A and 1B, a hair iron 100 is shown according to an example embodiment. Hair iron 100 includes an appliance such as a flat iron, straightening iron, curling iron, crimping iron, or other similar device that applies heat and pressure to hair in order to change the structure or appearance of the hair. Hair iron 100 has a housing 102 that forms the overall support structure of hair iron 100. The housing 102 may be composed of, for example, a plastic that is thermally insulative and electrically insulative and that possesses relatively high heat resistivity and dimensional stability and low thermal mass. Example plastics include polybutylene terephthalate (PBT) plastics, polycarbonate/acrylonitrile butadiene styrene (PC/ABS) plastics, polyethylene terephthalate (PET) plastics, including glass-filled versions of each. In addition to forming the overall support structure of hair iron 100, the housing 102 also provides electrical insulation and thermal insulation in order to provide a safe surface for the user to contact and hold during operation of hair iron 100.

Hair iron 100 further includes a pair of longitudinally extending arms 104, 106 that are movable between an open and closed position. Distal segments 108, 110 of arms 104, 106 are spaced apart from each other in the open position and are in contact, or close proximity with one another in the closed position. The arms clamshell or are pivotable relative to each other about a pivot axis 112 between the open position and the closed positions. Hair iron 100 may include a bias member (not shown), such as one or more springs, that biases one or both of arms 104, 106 toward the open position such that user actuation is required to overcome the bias applied to arms 104, 106 to bring arms 104, 106 together to the closed position. A lock 113 is provided to secure the arms in the closed position upon user manipulation.

Hair iron 100 includes a heater positioned on an inner side 114, 116 of one or both of arms 104, 106. Inner sides 114, 116 of arms 104, 106 include the portions of arms 104, 106 that face each other when arms 104, 106 are in the open and closed positions. In the example embodiment illustrated, each arm 104, 106 includes a respective heater 130, 132 opposed to one another on or within the arm 104, 106. Heaters 130, 132 supply heat to respective contact surfaces 118, 120 on arms 104, 106. Each contact surface 118, 120 is positioned on inner side 114, 116 of distal segment 108, 110 of the corresponding arm 104, 106. Contact surfaces 118, 120 may be formed directly by a surface of each heater 130, 132 or formed by a material covering each heater 130, 132, such as a shield or sleeve. Contact surfaces 118, 120 are positioned to directly contact and transfer heat to hair upon a user positioning hair between arms 104, 106 during use. Contact surfaces 118, 120 are positioned to mate against one another in a relatively flat orientation when arms 104, 106 are in the closed position in order to maximize the surface area available for contacting hair.

With reference to FIGS. 2A and 2B, hair iron 100′ (as seen in partial disassembled form) includes control circuitry 122 configured to control the power, thus the temperature, of each heater 130, 132. The control circuitry 122 in this design is bifurcated in the two arms of the hair iron, including a printed circuit board 133, 135 having a front (−f) and back (−b) sides. The PCB boards 133, 135 respectively relate to electrical circuit components for a power supply unit (PSU) and a microchip unit (MCU) that coordinate to selectively apply electrical current to the heaters 130, 132 (shown schematically in FIG. 3 ). The hair iron 100 further includes a power cord 124 for connecting hair iron 100 to an external line power or voltage source 126 to power the control circuitry 122 and heaters 130, 132. Amongst different geographies, the line power 126 is typically 115 VAC or 230 VAC. Regulating this power will be described below in relation to FIGS. 7A and 7B.

With reference to FIG. 3 , the regulation of line power 126 to the heaters 130, 132, includes control circuitry 122. A potentiometer 123 receives input from a user by twisting a handle 125 (FIG. 1A) of hair iron 100. The settings are varied, but twists of the handle generally relate to the hair iron 100 being off or powered on with high, medium, or low heat to the heaters 130, 132. A corresponding light emitting diode (LED) 125 indicates to the user whether or not the hair iron 100 is powered on or off. Other settings are possible. One or more triacs or switches 127 connect the heaters 130, 132 to line power 126 under control of the MCU 135. The MCU turns on the triacs 127 when the AC voltage of the line power is at or near a zero-crossing (ZC) as provided on a zero-crossing detection circuit supplied at 129 to the MCU. An accelerometer 131 detects manipulation of the hair iron 100 and the MCU will stop heating of the heaters 130, 132 regardless of the potentiometer 123 setting if the MCU does not receive any interrupts 133 per a given period of time, say every 60 seconds. In this way, the controller knows that a user is manipulating the hair iron for use and not merely setting it aside. The thermistors 172 gather current temperature readings of the heaters 130, 132 that the MCU uses to control the set-points, temperature increases, temperature decreases, and the like, of the heaters. The thermistors provided input to the MCU per a given period of time, such as per every 1 msec. Line voltage varies per geography, e.g., 115 VAC or 230 VAC, and such is detected by the PSU 133 and supplied to the MCU for controlling the power to the heaters based thereon as described below in FIGS. 7A and 7B.

In FIGS. 4A and 4B, heaters 130 or 132 are detailed to show them as removed from their housing. They may or may not be identical to one another. FIG. 4A shows inner face 151 of heater 130/132, and FIG. 4B shows outer face 150 of heater 130/132. In the embodiment illustrated, outer face 150 and inner face 151 are bordered by four sides or edges 152, 153, 154, 155 each having a smaller surface area than outer face 150 and inner face 151. In this embodiment, heater 130/132 includes a longitudinal dimension 156 that extends from edge 152 to edge 153 and a lateral dimension 157 that extends from edge 154 to edge 155. Heater 130/132 also includes an overall thickness 158 (FIG. 5 ) measured from outer face 150 to inner face 151.

Heater 130/132 includes one or more layers of a ceramic substrate 160, such as aluminum oxide (e.g., commercially available 96% aluminum oxide ceramic). Where heater 130/132 includes a single layer of ceramic substrate 160, a thickness of ceramic substrate 160 may range from, for example, 0.5 mm to 1.5 mm, such as 1.0 mm. Where heater 130 includes multiple layers of ceramic substrate 160, each layer may have a thickness ranging from, for example, 0.5 mm to 1.0 mm, such as 0.635 mm. In some embodiments, a length of ceramic substrate along longitudinal dimension 156 may range from, for example, 80 mm to 120 mm. In some embodiments, a width of ceramic substrate 160 along lateral dimension 157 may range from, for example, 15 mm to 24 mm, such as 17 mm or 22.2 mm. Ceramic substrate 160 includes an outer face 162 that is oriented toward outer face 150 of heater 130/132 and an inner face 163 that is oriented toward inner face 151 of heater 130/132. Outer face 162 and inner face 163 of ceramic substrate 160 are positioned on exterior portions of ceramic substrate 160 such that if more than one layer of ceramic substrate 160 is used, outer face 162 and inner face 163 are positioned on opposed external faces of the ceramic substrate 160 rather than on interior or intermediate layers of ceramic substrate 160.

In the example embodiment illustrated, outer face 150 of heater 130/132 is formed by outer face 162 of ceramic substrate 160 as shown in FIG. 4B. In this embodiment, inner face 163 of ceramic substrate 160 includes a series of one or more electrically resistive traces 164 and electrically conductive traces 166 positioned thereon. Resistive traces 164 include a suitable electrical resistor material such as, for example, silver palladium (e.g., blended 70/30 silver palladium). Conductive traces 166 include a suitable electrical conductor material such as, for example, silver platinum. In the embodiment illustrated, resistive traces 164 and conductive traces 166 are applied to ceramic substrate 160 by way of thick film printing. For example, resistive traces 164 may include a resistor paste having a thickness of 10-13 microns when applied to ceramic substrate 160, and conductive traces 166 may include a conductor paste having a thickness of 9-15 microns when applied to ceramic substrate 160. Resistive traces 164 form the heating element of heater 130 and conductive traces 166 provide electrical connections to and between resistive traces 164 in order to supply an electrical current to each resistive trace 164 to generate heat.

In the example embodiment illustrated, heater 130/132 includes a pair of resistive traces 164 a, 164 b that extend substantially parallel to each other (and substantially parallel to edges 154, 155) along longitudinal dimension 156 of heater 130. Heater 130 also includes a pair of conductive traces 166 a, 166 b that each form a respective terminal 168 a, 168 b of heater 130. Cables or wires 170 a, 170 b are connected to terminals 168 a, 168 b in order to electrically connect resistive traces 164 and conductive traces 166 to, for example, control circuitry 122 and voltage source 126 in order to selectively close the circuit formed by resistive traces 164 and conductive traces 166 to generate heat. Conductive trace 166 a directly contacts resistive trace 164 a, and conductive trace 166 b directly contacts resistive trace 164 b. Conductive traces 166 a, 166 b are both positioned adjacent to edge 152 in the example embodiment illustrated, but conductive traces 166 a, 166 b may be positioned in other suitable locations on ceramic substrate 160 as desired. In this embodiment, heater 130/132 includes a third conductive trace 166 c that electrically connects resistive trace 164 a to resistive trace 164 b. Portions of resistive traces 164 a, 164 b obscured beneath conductive traces 166 a, 166 b, 166 c in FIG. 4A are shown in dotted line. In this embodiment, current input to heater 130/132 at, for example, terminal 168 a by way of conductive trace 166 a passes through, in order, resistive trace 164 a, conductive trace 166 c, resistive trace 164 b, and conductive trace 164 b where it is output from heater 130 at terminal 168 b. Current input to heater 130 at terminal 168 b travels in reverse along the same path.

In some embodiments, heater 130/132 includes a thermistor 172 positioned in close proximity to a surface of heater 130/132 in order to provide feedback regarding the current temperature of heater 130/132 to control circuitry 122. In some embodiments, thermistor 172 is positioned on inner face 163 of ceramic substrate 160. In the example embodiment illustrated, thermistor 172 is welded directly to inner face 163 of ceramic substrate 160. In this embodiment, heater 130/132 also includes a pair of conductive traces 174 a, 174 b that are each electrically connected to a respective terminal of thermistor 172 and that each form a respective terminal 176 a, 176 b. Cables or wires 178 a, 178 b are connected to terminals 176 a, 176 b in order to electrically connect thermistor 172 to, for example, control circuitry 122 in order to provide closed loop control of heater 130. In the embodiment illustrated, thermistor 172 is positioned at a central location of inner face 163 of ceramic substrate 160, between resistive traces 164 a, 164 b and midway from edge 152 to edge 153. In this embodiment, conductive traces 174 a, 174 b are also positioned between resistive traces 164 a, 164 b with conductive trace 174 a positioned toward edge 152 from thermistor 172 and conductive trace 174 b positioned toward edge 153 from thermistor 172. However, thermistor 172 and its corresponding conductive traces 174 a, 174 b may be positioned in other suitable locations on ceramic substrate 160 so long as they do not interfere with the positioning of resistive traces 164 and conductive traces 166.

FIG. 5 is a cross-sectional view of heater 130/132 taken along line 5-5 in FIG. 4A. With reference to FIGS. 4A, 4B and 5 , in the embodiment illustrated, heater 130/132 includes one or more layers of printed glass 180 on inner face 163 of ceramic substrate 160. In the embodiment illustrated, glass 180 covers resistive traces 164 a, 164 b, conductive trace 166 c, and portions of conductive traces 166 a, 166 b in order to electrically insulate such features to prevent electric shock or arcing. The borders of glass layer 180 are shown in dashed line in FIG. 4A. In this embodiment, glass 180 does not cover thermistor 172 or conductive traces 174 a, 174 b because the relatively low voltage applied to such features presents a lower risk of electric shock or arcing. An overall thickness of glass 180 may range from, for example, 70-80 microns. FIG. 5 shows glass 180 covering resistive traces 164 a, 164 b and adjacent portions of ceramic substrate 160 such that glass 180 forms the majority of inner face 151 of heater 130/132. Outer face 162 of ceramic substrate 160 is shown forming outer face 150 of heater 130/132 as discussed above. Conductive trace 166 c, which is obscured from view in FIG. 5 by portions of glass 180, is shown in dotted line. FIG. 5 depicts a single layer of ceramic substrate 160. However, ceramic substrate 160 may include multiple layers as depicted by dashed line 182 in FIG. 5 .

Heater 130/132 may be constructed by way of thick film printing. For example, in one embodiment, resistive traces 164 are printed on fired (not green state) ceramic substrate 160, which includes selectively applying a paste containing resistor material to ceramic substrate 160 through a patterned mesh screen with a squeegee or the like. The printed resistor is then allowed to settle on ceramic substrate 160 at room temperature. The ceramic substrate 160 having the printed resistor is then heated at, for example, approximately 140-160 degrees Celsius for a total of approximately 30 minutes, including approximately 10-15 minutes at peak temperature and the remaining time ramping up to and down from the peak temperature, in order to dry the resistor paste and to temporarily fix resistive traces 164 in position. The ceramic substrate 160 having temporary resistive traces 164 is then heated at, for example, approximately 850 degrees Celsius for a total of approximately one hour, including approximately 10 minutes at peak temperature and the remaining time ramping up to and down from the peak temperature, in order to permanently fix resistive traces 164 in position. Conductive traces 166 and 174 a, 174 b are then printed on ceramic substrate 160, which includes selectively applying a paste containing conductor material in the same manner as the resistor material. The ceramic substrate 160 having the printed resistor and conductor is then allowed to settle, dried and fired in the same manner as discussed above with respect to resistive traces 164 in order to permanently fix conductive traces 166 and 174 a, 174 b in position. Glass layer(s) 180 are then printed in substantially the same manner as the resistors and conductors, including allowing the glass layer(s) 180 to settle as well as drying and firing the glass layer(s) 180. In one embodiment, glass layer(s) 180 are fired at a peak temperature of approximately 810 degrees Celsius, slightly lower than the resistors and conductors. Thermistor 172 is then mounted to ceramic substrate 160 in a finishing operation with the terminals of thermistor 172 directly welded to conductive traces 174 a, 174 b.

Thick film printing resistive traces 164 and conductive traces 166 on fired ceramic substrate 160 provides more uniform resistive and conductive traces in comparison with conventional ceramic heaters, which include resistive and conductive traces printed on green state ceramic. The improved uniformity of resistive traces 164 and conductive traces 166 provides more uniform heating across contact surface 118 as well as more predictable heating of heater 130.

Preferably, heaters 130/132 are produced in an array for cost efficiency. Heaters are separated into individual heaters 130/132 after the construction of all heaters is completed, including firing of all components and any applicable finishing operations. In some embodiments, individual heaters are separated from the array by way of fiber laser scribing. Fiber laser scribing tends to provide a more uniform singulation surface having fewer microcracks along the separated edge in comparison with conventional carbon dioxide laser scribing.

It will be appreciated that the example embodiments illustrated and discussed above are not exhaustive and that the heater of the present disclosure may include resistive and conductive traces in many different geometries, including resistive traces on the outer face and/or the inner face of the heater, as desired. Other components (e.g., a thermistor) may be positioned on either the outer face or the inner face of the heater as desired.

The present disclosure does, however, provide a ceramic heater having a low thermal mass in comparison with the heaters of conventional hair irons. In particular, thick film printed resistive traces on an exterior face (outer or inner) of the ceramic substrate provides reduced thermal mass in comparison with resistive traces positioned internally between multiple sheets of ceramic. The use of a thin film, thermally conductive sleeve, such as a polyimide sleeve) also provides reduced thermal mass in comparison with metal holders, guides, etc. The low thermal mass of the ceramic heater of the present disclosure allows the heater, in some embodiments, to heat to an effective temperature for use in a matter of seconds (e.g., less than 5 seconds), significantly faster than conventional hair irons. The low thermal mass of the ceramic heater of the present disclosure also allows the heater, in some embodiments, to cool to a safe temperature after use in a matter of seconds (e.g., less than 5 seconds), again, significantly faster than conventional hair irons.

Further, embodiments of the hair iron of the present disclosure operate at a more precise and more uniform temperature than conventional hair irons because of the closed loop temperature control provided by the thermistor in combination with the relatively uniform thick film printed resistive and conductive traces. The low thermal mass of the ceramic heater and improved temperature control permit greater energy efficiency in comparison with conventional hair irons. The rapid warmup and cooldown times of the ceramic heater of the present disclosure also provide increased safety by reducing the amount of time the hair iron is hot but unused. The improved temperature control and temperature uniformity further increase safety by reducing the occurrence of overheating. The improved temperature control and temperature uniformity also improve the performance of the hair iron of the present disclosure.

Referring now to FIG. 6A, a method 185 is provided for determining the AC line voltage. At 187, both the heaters (130, 132) are set to operate at a given setpoint temperature (T0°) well underneath their operating temperature. If the operating temperature is 200° C., for example, then a representative setpoint temperature is about 150° C. At 189, the thermistors (172) read the initial or current temperatures (T1°) of the heaters and at 191 a corresponding timer is initialized (t0) and begins its count. At the same time, at 193, current is allowed to heat the heaters to begin heating up to their setpoint temperatures, but in a manner whereby only 50% of the power is turned on. This also occurs in a manner whereby the controller only allows half-cycles of the AC line voltage to heat the heaters, say five out of ten half-cycles, or 50%. In this way, the controller keeps the heaters from overheating and prevents cracking. At 195, the controller determines whether the current temperature T1° is less than or equal to the setpoint temperature T0° less 20° C. (i.e., is T1°≤T0°−20° C.?). If so, the warmup time (WUT) of each heater is calculated at 199. If not, the current temperature of the heater is already much closer to the setpoint temperature and so the line voltage that powers the hair iron is calculated in addition to calculating the warmup time of the heaters at 197. At 201, the controller continues monitoring of the temperature of the heaters and a time or timer count (t1) is determined when the current temperature is ten degrees more than initially read (i.e., T1°+10° C.). Similarly, the controller monitors the temperature of the heaters at 203 and the time or timer count (t2) is noted when the current temperature is twenty degrees more than initially read (i.e., T1°+20° C.). Then, at 205, the warmup time (WUT) of a given heater is the time or timer count difference between t2 and t1 (i.e., t2−t1).

Once known, the controller operates the heaters under a scheme of either low or high voltage control at 207. Intuitively, if the line voltage corresponds to a low power source, such as 115 VAC, the time to heat the heaters from their setpoint will be slower than if the line voltage corresponds to a high-power source, such as 230 VAC, and vice versa. It is expected that low voltage control occurs for AC line voltages between 90-140 VAC and high voltage control occurs above that. In a further embodiment, if the warmup time t2−t1 is less than or equal to 255 msec (t2−t1≤255 milliseconds), a line voltage bit is set to “1” for high voltage temperature control. Conversely, if the warmup time t2−t1 is greater than 334 msec (t2−t1>334 milliseconds), the line voltage bit is set to 0 for low voltage temperature control. The below Table shows this pictorially.

Heater Current Measured WUT Power Voltage WUT > WUT < limit Bit 334 msec 255 msec 50% 0\LV Take No Reset voltage Action bit to 1 50% 1\HV Reset voltage Take No bit to 0 Action

After finishing line voltage detection 185, the AC manager of the controller will immediately switch from line voltage detection to actual control of the temperature of the heaters 130, 132. Based on the temperature difference of the measured heater temperature by the thermistor and the setpoint temperature, the PID (proportional-integral-derivative) controller will calculate a temperature response to the current temperature to set the required heating power for each heater. The controller will adjust PID gains in a manner to minimize warm up time, reduce ramp up temperature overshoot, and achieve tight steady state temperature control. In general, however, if the current heater temperature is more than 30° C. lower than the setpoint temperature, ramping up PID gains will be used. Otherwise, steady state PID gains will be selected.

Referring now to FIG. 6B, an example method 200 for controlling power delivered to the heaters 130, 132 is illustrated. In it, a determination 202 of the line voltage corresponds to either low power, such as 115 VAC, or high power, such as 230 VAC. If the voltage is low power, there exists no special power restrictions 204 on the heaters 130, 132 and flicker does not represent a sizable concern. If high power exists, on the other hand at 206, the heating power to the heaters is capped at 25% capacity. Once there, the PID output power or temperature response is determined. If the PID output power is less than 10% at 208, meaning the current temperature is relatively close to the desired temperature of the heaters, then the heaters are turned off at 210. If the PID output power at 208 is equal to or more than 10%, but less than 20%, the heaters 130, 132 are multiplexed (MUX) together in a manner at 212 whereby their heater power turned on for four of twenty-four half-cycles of AC power, or 4/24=16.7% (FIG. 7A, described below). If the PID output power at 208 is greater than or equal to 20%, the heaters 130, 132 are multiplexed together in a manner at 214 whereby their heater power is turned on for six of twenty-four half-cycles of AC power, or 6/24=25% (FIG. 7B, described below).

With reference to FIGS. 7A and 7B, when situations dictate that the heaters 130, 132 become multiplexed, neither heater is powered on at the same time as the other heater and powering on and off occurs at zero-crossings of the AC waveform to reduce flicker and harmonic components. In either situation, the waveform of the AC line voltage is divided into 24 half-cycles of AC power (numbered 1-24). A first twelve of the 24 half-cycles are reserved for one heater while the last twelve cycles are reserved for the other heater. Line 250 demarks the first and last twelve cycles from one another. Thereafter, in a repeating pattern, current becomes applied to the first heater 130 during the first twelve cycles while current is next applied to the opposite heater 132 during the next twelve cycles, or vice versa. As seen, solid lines 260-x, 270-y represent the application of current to the heaters thereby powering them on, whereas the dashed lines correspond to no application of current to the heaters. Thus, heater 132 is off at 264 during the second twelve half-cycles of AC power when heater 130 is powered on at during the first twelve half-cycles of AC power. Conversely, heater 130 is off at 262 during the first twelve half-cycles of AC power when heater 132 is powered on at during the second twelve half-cycles of AC power. That either heater in FIG. 7A is only powered on for four of the twenty-four half-cycles of AC power (260-1, 260-2, 260-3, 260-4 corresponding to heater 130 and 260-5, 260-6, 260-7, 260-8 corresponding to heater 132), only 16.7% power levels of the heaters are achieved in the hair iron, or 4/24=16.7%. Similarly, in FIG. 7B, that either heater is only powered on for six half-cycles of the twenty-four half-cycles of AC power (270-1, 270-2, 270-3, 270-4, 270-5, 270-6 corresponding to heater 130 and 270-7, 270-8, 270-9, 270-10, 270-11, 270-12 corresponding to heater 132), only 25% power levels of the heaters are achieved in the hair iron, or 6/24=25%. Of course, other percentages of power levels are possible, as are different half-cycles of AC power.

As examples, with reference back to FIG. 3 , artisans will note that the heaters 130, 132 are two independent heating elements of equal resistance and each has a current temperature feedback mechanism by way of the thermistor 172 to the controller 135. During use, the controller activates the switch 127 to control AC power delivery to the heaters. Using the AC zero-crossing feedback 129, the power delivery is synchronized precisely with the zero-crossings of the AC mains voltage waveform. This establishes the minimum unit of power delivery as a single half-cycle of the AC waveform. The controller modulates the current of each heater to achieve a desired temperature. This action is moderated by a temperature control loop (e.g. PID) running on the controller. That is, the control loop calculates a desired temperature response by way of a power level in units of percent, where 100% is equal to rated wattage of the heater. The fundamental period of heater power delivery in the following embodiments is eight half-cycles.

With reference to the table 300 of FIG. 8 , the controller will cause the switch to connect heaters to the AC line voltage for an integer number of half-cycles within this period. To achieve a power level percent (%) of 12.5% (e.g., 1/8×100%), for example, one AC half-cycle 302 of one-thru-eight total half-cycles 301 of power is turned on to heat the heater. Similarly, to achieve a power level percent of 50%, four AC half-cycles 304 of one-thru-eight total half-cycles 301 of power are turned on to the heat the heater (e.g., 4/8×100%). Similarly, too, all power level percentages of the heaters is read from the table 300, e.g., power level percentages 0%, 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5%, and 100%. However, depending on the exact number and placement of ON half-cycles in the one-thru-eight half-cycle period (e.g., the “pattern”), there arises a DC offset in the resultant current waveform. Namely, if there exists an odd number 1, 3, 5, and 7 of AC half-cycles of power turned ON in the one-thru-eight AC half-cycles of power, corresponding to power level percentages 12.5%, 37.5%, 62.5%, and 87.5%, respectively, a DC offset 306, 308, 310, and 312 results, respectively. Conversely, if the number of positive-polarity half-cycles equals the number of negative-polarity half-cycles, or there exists zero or an even number 0, 2, 4, 6, and 8, of AC half-cycles of power turned ON in the one-thru-eight AC half-cycles of power, corresponding to power level percentages 0%, 25%, 50%, 75%, and 100%, respectively, a DC offset does not exist. Stated mathematically, the DC offset 320 for a given pattern can be quantified by taking the difference between positive-polarity and negative-polarity ON half-cycles, or DC Offset=Number of positive AC half-cycles−Number of negative AC half-cycles. Importantly, the inventors observe that when the total length of the pattern is an even number of AC half-cycles, if the pattern is reversed the pattern sequence will yield a new pattern whose DC offset is equal in magnitude yet opposite in sign. This is because positive half-cycles will be replaced by negative half-cycles and vice versa. In turn, three Cases 1, 2, and 3 arise whereby the PID controllers calculate a desired temperature response for its given heater whereby the power level percent to be applied to the heaters has both power level percentages with a DC offset of 1 (Case 1); has both power level percentages with a DC offset of 0 (Case 2); and has one power level percent for one heater with a DC offset of 1 and one power level percent for the other heater with a DC offset of 0 (Case 3).

For Case 1, with further reference to FIG. 9A, if a desired temperature response calculated by the PID controller for the heaters 130, 132 is both a power level percent having a DC offset, such as power level 37.5%, where there exists three ON AC half-cycles of power, the controller switches the AC half-cycles of power to the heaters 130, 132 in a manner that results in a combined DC offset of 330 of zero. That is, there exists a first 332 AC half-cycle of power of 37.5% applied to the heater 130 that is a reversed pattern of a second 333 AC half-cycle of power of 37.5% applied to the heater 132. As the former has a DC offset 335 of +1, while the latter has a DC offset 337 of −1, there exists zero DC offset 330 applied to the combined, both heaters 130, 132. This leverages the fact that the resistances of the heaters 130, 132 are equal to one another and therefor the current amplitude to both heaters is equal.

For Case 2, if a given a desired temperature response calculated by the PID controller for the heaters 130, 132 is both a power level percent having no DC offset, or zero DC current, a trivial case exists and the controller merely switches the AC half-cycles of power to the heaters 130, 132 according to the table 300.

For Case 3, the situation exists whereby a desired temperature response calculated by the PID controller for the heaters 130, 132 has both a DC offset for one of the heaters 130 or 132, but not both of the heaters 130 and 132. If summing together the two AC waveforms cannot result in a combined DC offset of zero, such as at 330 (FIG. 9A), FIG. 9B teaches the extension of the total pattern period of AC half-cycles to sixteen (16) AC half-cycles, instead of eight. In turn, the first eight AC half-cycles 340, the AC waveform pattern applied to one of the heaters is provided in the forward direction and the second eight AC half-cycles 342 is provided to the other or opposite heater is in the reverse direction, or vice versa. Since the forward and reverse of a given pattern have equal and opposite DC offsets, the complete waveform has zero DC bias 344 over the sixteen half-cycle period. Of course, these three cases could be changed to more or fewer AC half-cycles of power.

Advantages should be now readily apparent to those skilled in the art. Among them, the hair iron herein: 1) can be conveniently used anywhere in the world without needing a voltage converter; 2) successfully meets IEC flicker and harmonic requirements, while many competitive products cannot; and 3) delivers thermal performance that readies itself in as few as five seconds, with fast cooling times.

The foregoing description illustrates various aspects of the present disclosure. It is not intended to be exhaustive. Rather, it is chosen to illustrate the principles of the present disclosure and its practical application to enable one of ordinary skill in the art to utilize the present disclosure, including its various modifications that naturally follow. All modifications and variations are contemplated within the scope of the present disclosure as determined by the appended claims. Relatively apparent modifications include combining one or more features of various embodiments with features of other embodiments. 

1. A hair iron, comprising: two longitudinally extending arms each having a heater corresponding to an opposite heater on an opposite arm of the two longitudinally extending arms, wherein during use hair becomes placed between said heater and opposite heater to heat the hair, said heater and opposite heater being a ceramic heater and each having a setpoint temperature for low and high voltage line voltage conditions; at least one temperature sensor to sense a temperature of said heater and opposite heater; an electrical cord to connect said heater and opposite heater to a line voltage during use; a controller in communication with the temperature sensor and said heater and opposite heater; and a line voltage detector between the cord and the controller, an output of the line voltage detector being supplied to the controller, wherein the controller is configured to limit heating power to said heater and opposite heater to about fifty percent or less before the line voltage detector detects the line voltage.
 2. The hair iron of claim 1, further wherein the controller is configured to read from the at least one temperature sensor a current temperature of said heater and opposite heater and initializing a timer to time zero t0.
 3. The hair iron of claim 2, further wherein the controller is configured to calculate a warmup time for said heater and opposite heater, the warmup time corresponding to a second time t2 less a first time t1, wherein the second time t2 corresponds to a second time when a second read temperature from the at least one temperature sensor is the current temperature plus 20° C. and the first time t1 corresponds to a first time when a first read temperature from the at least one temperature sensor is the current temperature plus 10° C.
 4. The hair iron of claim 3, wherein if the warm up time is less than a first predetermined time, the controller is further configured to initiate a high voltage control routine of said heater and opposite heater.
 5. The hair iron of claim 4, wherein the first predetermined time is about 255 msec.
 6. The hair iron of claim 3, wherein if the warm up time is greater than a second predetermined time, the controller is further configured to initiate a low voltage control routine of said heater and opposite heater.
 7. The hair iron of claim 6, wherein the second predetermined time is about 344 msec.
 8. A hair iron, comprising: two longitudinally extending arms each having a heater corresponding to an opposite heater on an opposite arm of the two longitudinally extending arms, wherein during use hair becomes placed between said heater and opposite heater to heat the hair, said heater and opposite heater being a ceramic heater; a line voltage detector to detect whether a line voltage supplied through a cord is either of a low voltage or high voltage; and a controller to coordinate heating of said heater and opposite heater, wherein if said high voltage exists, the controller being configured to alternate heating between said heater and opposite heater and heating only one of said heater and opposite heater at any time.
 9. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a first twelve of said half-cycles of power, the controller prevents said opposite heater from receiving heating current.
 10. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a first twelve of said half-cycles of power, the controller allows said heater to receive heating current for only four of said half-cycles of power.
 11. The hair iron of claim 10, wherein said four of said half-cycles of power include two positive and two negative said half-cycles of power.
 12. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a second twelve of said half-cycles of power, the controller prevents said heater from receiving heating current.
 13. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a second twelve of said half-cycles of power, the controller allows said opposite heater to receive heating current for only four of said half-cycles of power.
 14. The hair iron of claim 13, wherein said four of said half-cycles of power include two positive and two negative said half-cycles of power.
 15. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a first twelve of said half-cycles of power, the controller allows said heater to receive heating current for only six of said half-cycles of power.
 16. The hair iron of claim 15, wherein said six of said half-cycles of power include three positive and three negative said half-cycles of power.
 17. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a second twelve of said half-cycles of power, the controller allows said opposite heater to receive heating current for only six of said half-cycles of power.
 18. The hair iron of claim 17, wherein said six of said half-cycles of power include three positive and three negative said half-cycles of power.
 19. The hair iron of claim 8, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, wherein during a second twelve of said half-cycles of power, the controller prevents said heater from receiving heating current.
 20. A hair iron, comprising: two longitudinally extending arms each having a heater corresponding to an opposite heater on an opposite arm of the two longitudinally extending arms, wherein during use hair becomes placed between said heater and opposite heater to heat the hair, said heater and opposite heater being a ceramic heater; and a controller to coordinate heating of said heater and opposite heater, wherein if a high voltage is supplied to the hair iron, the controller is configured to alternate heating between said heater and opposite heater and heating only one of said heater and opposite heater at any time, wherein the high voltage includes an alternating current sinusoidal waveform, the waveform having consecutive half-cycles of power, the controller said heating said only one of said heater and opposite heater at 16.7% or 25% of a consecutive twelve of said half-cycles of power. 